air- and moisture-stable complexes; high catalytic performance; general catalytic cycle

Abstract

The syntheses and characterization of a series of (NHC)Pd(II)
complexes (NHC = N-heterocyclic carbene) are described. A
variety of architectures and precursors have been employed to
lead to numerous air- and moisture-stable complexes. The use of
those complexes as pre-catalysts in cross-coupling (Suzuki-
Miyaura, Buchwald-Hartwig) and related (catalytic
dehalogenation, £-ketone arylation) reactions is also discussed. A
comparison of the activity of a variety of (NHC)Pd complexes as
pre-catalysts for cross-coupling reactions was carried out. The
results indicate that the activation of those pre-catalysts, leading to
the catalytically active [(NHC)Pd(0)] species, was key in assuring
high catalytic performance under mild reaction conditions. For the
first series of complexes described, (NHC)Pd(allyl)Cl complexes,
a better understanding of the process leading to the catalytically
active species has permitted us to introduce simple modifications
(alkyl or aryl groups at the allyl moiety) that dramatically alter the
performance of the complexes by facilitating their activation,
decreasing reaction times, catalyst loadings and even allowing
reactions to be conducted at room temperature. Catalyst loadings
as low as 0.05 mol% can be used for the Suzuki-Miyaura
crosscoupling of aryl chlorides and aryl boronic acids at room
temperature, leading to the synthesis of poly-ortho-substituted
biaryls in excellent yields. This catalyst loading is the lowest ever
used for this purpose. The system also allows for the first
examples of coupling between aryl chlorides and alkenyl boronic
acids at room temperature. When the temperature is raised to 80 ¢
XC for these reactions, a catalyst loading as low as 50 ppm can be
used to effectively carry out Suzuki-Miyaura cross-couplings in
remarkably short reaction times. As an added advantage, these
complexes are air- and moisture-stable and can be prepared in a
facile one-pot, multigram scale synthesis from commercially
available starting materials in very high yields. The second series
of complexes described revolves around the (NHC)Pd(acac)n
framework. These complexes are also air- and moisture-stable and
can be prepared in a one-step synthesis in high yields from
commercially available materials. These complexes were tested
forƒn £-ketone arylation and Buchwald-Hartwig amination
reactions affording high yields of the desired products, in short
reaction times and mild reaction conditions.

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